Oscillator circuits play quite an important part in the field of electronics circuits, including communication systems, providing clocking and timing signals in digital circuits and frequency translation in radio frequency (RF) circuits.
Among those oscillators, the well-known Colpitts oscillator is widely used in the field. An example of a Colpitts oscillator is illustrated in FIG. 1A showing a transistor M biased by a current source I and associated with an inductance L and the series combination of C1 and C2 capacitors forming the parallel resonant tank circuit defining the oscillation frequency, in accordance with the formula.
  f  =      1          2      ⁢      π      ⁢                        L          ⁡                      (                                                            C                  1                                ⁢                                  C                  2                                                                              C                  1                                +                                  C                  2                                                      )                              
In the Colpitts circuits, the arrangement of a variable frequency oscillator (VFO) requires the voltage divider to include a variable capacitor (either C1 or C2), thus causing the feedback voltage to be variable as well.
A variant of the Colpitts circuit, the well-known Clapp oscillator, avoids this problem by using two fixed capacitors C1 and C2, with a variable Capacitor which is arranged in series with the inductor, as shown in FIG. 2A, and which oscillates at the following frequency:
  f  =            1              2        ⁢        π              ⁢                            1          L                ⁢                  (                                    1                              C                0                                      +                          1                              C                1                                      +                          1                              C                2                                              )                    
The Clapp oscillator is quite often used in association with a varactor, as illustrated in FIG. 2, which is based on a reversely biased diod forming the variable capacitor.
There are other oscillation circuits well-known in the art, and particular those based on a differential topology, such as the well-known cross-coupled differential architecture.
Generally speaking, all those conventional architectures are sensitive to parasitic capacitances which might impair the possibility of achieving, at the same time, a wide tuning range together with a low noise.
Therefore, there is a desire for a new topology of a variable frequency oscillator (F) or a voltage controlled oscillator (VCO) which achieves those goals.